[GDBserver] Replace "reinsert_breakpoint" with "single_step_breakpoint"

reinsert_breakpoint is used for software single step, so it is more
clear to rename it to single_step_breakpoint.  This was pointed out in
the review https://sourceware.org/ml/gdb-patches/2016-05/msg00429.html
I don't rename "other_breakpoint" in this patch.

gdb/gdbserver:

2016-09-02  Yao Qi  <yao.qi@linaro.org>

	* linux-low.c: Replace "reinsert_breakpoints" with
	"single_step_breakpoints".  Replace "reinsert breakpoints"
	with "single-step breakpoints".
	* mem-break.c: Likewise.
	* mem-break.h: Likewise.